1. Field of the Invention
The present invention generally relates to a developing apparatus and an image forming method each used for an image forming apparatus such as a copying machine or a printer. In particular, the present invention relates to a developing apparatus and an image forming method in which colored toner and transparent toner are used by a two-component development mode.
2. Related Background Art
Conventionally, a two-component development mode in which non-magnetic toner and magnetic carrier are mixed and used as a developer has been widely used in an image forming apparatus employing an electrophotographic mode, in particular, an image forming apparatus performing the formation of a chromatic color image.
The two-component development mode has merits over any other development mode currently adopted such as the stability of image quality and the durability of an apparatus against long-term use. However, the deterioration of the developer due to the long-term use, in particular, a reduction in charge amount (hereinafter referred to as “triboelectricity”) of toner due to the deterioration of the carrier causes a change in developability. As a result, for example, an image failure such as a fluctuation in tint or toner scattering occurs with increasing number of sheets on which images are outputted. Therefore, the long-term use of the image forming apparatus requires a downtime (a time period for which the image output cannot be performed owing to, for example, the adjustment of the apparatus) and labor for replacing a developer.
In view of the foregoing, Japanese Patent Examined Publication No. H02-021591 has proposed a method of saving labor for replacing the developer while maintaining the performance of the developer to some extent involving: gradually collecting the deteriorated developer; and newly replenishing the developer in an amount corresponding to the amount of the collected developer. In other words, the deteriorated developer (carrier) is gradually replaced with a new one, with the result that the following advantage can be obtained. That is, the apparent progress of the deterioration of the carrier stops, the properties of the entire developer can be stabilized, and the automatic replacement of the developer can eliminate an operation of replacing the developer.
In particular, in recent years, in electrophotography using the two-component development mode, there has been a growing demand on the output of a stable image with a downtime reduced as much as possible in, for example, a POD market. A technique for reducing the downtime as proposed in Japanese Patent Examined Publication No. H02-021591 is useful in meeting the demand. Furthermore, the deterioration of the developer can be stabilized at a certain level, so the fluctuation in image quality due to the deterioration of the developer can be prevented.
The deterioration of the carrier can be represented as the reduction in ability of the carrier to provide toner with triboelectricity. To be specific, an ability to provide triboelectricity gradually reduces owing to the shaving of a coating agent with which the carrier is coated and the adhesion of toner/an external additive to the surface of the carrier, so the carrier deteriorates.
The technique proposed in Japanese Patent Examined Publication No. H02-021591 enables the deterioration of the carrier in the developer container to be prevented. This is because the level at which the carrier deteriorates can be changed depending on the frequency of the replenishment/discharge of the carrier per number of outputted sheets.
Simply speaking, the increase in frequency of the replacement of the carrier causes the developer to be stably present in a state with improved freshness. Here, the variation in level at which the carrier deteriorates with the image ratio will be further described.
A time period for which each carrier is used in the developer container is represented as an “age” in a unit of number of A4-size recording materials on each of which the image is outputted. Here, let P(x) and W(g) be the average age of carriers in the developer container at a certain number x of sheets on which images are outputted in a durability test and the total amount of the carriers in the developer container, respectively. In addition, when one image is additionally formed, d(g) of a new carrier is replenished in accordance with the consumption of toner, while d(g) of the developer present in the developer container is similarly discharged.
For convenience of calculation, suppose that image formation and the replacement of the carrier are performed serially in a time series. Let P(x) and Q(x) be the average age of carriers immediately after the formation of x images and immediately before the replacement of the carrier and the average age of the carriers immediately after the replacement of the carrier, respectively. Therefore, the following expression can be obtained.Q(x)=P(x)×[(W−d)/W]+P(0)×[d/W]  (1)Here, P(0)=0 because P(0) is an initial average age. Therefore, the following expression can be obtained.Q(x)=P(x)×[(W−d)/W]  (2)
P(x+1) corresponds to a state where one image is additionally formed in a state where the average age is Q(x). Since carriers are supposed to be equally used for image formation during a period commencing on the state where the average age is Q(x) and ending on the state represented by P(x+1), the following expression can be obtained.P(x+1)=Q(x)+1  (3)Combining the expressions (2) and (3) yields the following expression.P(x+1)=P(x)×[(W−d)/W]+1  (4)That is, the following expression can be obtained.P(x)=[1−(1−d/W)x]×W/d  (5)
In other words, the average age of the carriers after the automatic replacement of the developer converges on W/d (=the total amount of the carriers in the developer container/the amount of the replaced carrier per sheet).
To be specific, when the amount of the developer in a developer container is 375 g and the toner concentration of the developer in the developer container (a ratio of the weight of toner to the total weight of the developer: hereinafter referred to as the “TD ratio”) is 8%, the amount of the carrier is about 345 g. Then, the weight ratio of the carrier of the developer for replenishment (hereinafter referred to as the “replenishment developer”) (the ratio of the carrier weight to the total weight of the developer: hereinafter referred to as the “CD ratio”) is 15%. When the bearing amount of toner for outputting a maximum concentration is 0.7 mg/cm2, 21.3 mg of toner per A4-size recording material is used in the case where an image ratio is 5%. At the same time, the amount of the replaced carrier per sheet is 3.8 mg. The results of calculation on the basis of the foregoing are represented in FIG. 3 as a graph showing the transition of an average age.
Data indicated by a broken line in the drawing show results in the case where the CD ratio of the replenishment developer is 0%, that is, the amount of the mixed carrier is 0. In the line, the number of sheets is equal to the average age of carriers. FIG. 3 shows the results in the case where the image ratio is 10% and the results in the case where the image ratio is 50% as well.
As is apparent from FIG. 3, the use of the replenishment developer having a CD ratio of 15% causes the average age of carries to saturate at 90K sheets at the time point when the number of sheets on which images are outputted each having an image ratio of 5% is 300K (300,000). In contrast, the use of the replenishment developer having a CD ratio of 0% causes the average age of carries to saturate at 300K sheets at the same time point, with the result that the replacement of a developer is forced.
The level at which the carrier in the developer container deteriorates can be suppressed by discharging the carrier from the developer container, and, at the same time, replenishing the carrier upon replenishment with toner as described above.
Meanwhile, for additionally improving image quality, a technique for improving image quality has been proposed, which involves fixing, for example, transparent toner to an uppermost layer to improve the gloss of a surface.
For example, Japanese Patent Application Laid-Open No. H04-278967 proposes a technique for providing a color image having a color tone close to that of silver halide photography involving performing development by means of transparent toner on the entire surface of a region where an image can be formed to improve the gloss of an image surface.
In addition, Japanese Patent Application Laid-Open No. H05-006033, Japanese Patent Application Laid-Open No. H05-127437, and Japanese Patent Application Laid-Open No. 2000-147863 each propose a method of forming an image which has reduced irregularities due to toner built-up and is closer to silver halide photography involving: performing development by means of transparent toner on the entire surface of a region where an image can be formed to improve gloss; and adjusting the bearing amount of the transparent toner to form a surface having uniform surface property and equal texture.
However, the use of colored toner and transparent toner in a two-component development mode is found to cause such problem as described below.
In other words, when development by means of the transparent toner is performed on the entire surface of a region where an image can be formed, the image ratio of the transparent is much larger than that of the colored toner. Therefore, only in the case of the transparent toner, a toner consumption per image increases, and the number of times of replenishment with a developer also increases owing to the increase. As a result, the amount of a carrier with which a developing device is replenished also increases. Therefore, the frequency of the replacement of only a carrier for the transparent toner increases, and, when a cost for the carrier is high, a running cost increases in some cases.
That is, a high image ratio is disadvantageous in terms of running cost although a developer is stably present in a state with improved freshness because the frequency of the replacement of a carrier is high.
As can be seen from FIG. 3, the average age of carriers in the case where an image ratio is 5% is 10 times as long as that in the case where an image ratio is 50%. In other words, a cost for a carrier in the case where an image ratio is 50% is 10 times as high as that in the case where an image ratio is 5%.
In addition, the degree to which a carrier for colored toner deteriorates in association with an increase in number of sheets on which images are outputted does not coincide with the degree to which a carrier for transparent toner deteriorates in association with an increase in number of sheets on which images are outputted. Therefore, a balance between the bearing amount of the colored toner and that of the transparent toner is lost with increasing number of sheets on which images are outputted, so gloss and smoothness are lost in some cases. In other words, the thickness of only the toner of a colored portion increases with increasing number of sheets on which images are outputted, so gloss and smoothness are lost in some cases.